1. Field of the Invention
This invention relates to a gene coding for so-called signal peptides. More particularly, the present invention pertains to a DNA fraction of a gene coding for a signal peptide, which is to be used in a host-vector system employing a bacterium belonging to the genus Streptomyces as a host by incorporation in the vector DNA.
2. Background of the Field
By means of bioengineering techniques which have rapidly progressed in recent years, it has become possible to cause hormones such as insulin and growth hormones, vaccines such as that of hepatitis B virus, lymphokines such as interferon and interleukin, and other protein products, all of which had heretofore been produced by higher organisms, to be produced by microorganisms.
Production of such a protein by a geneticaly engineered microorganism is generally practiced according to a process comprising combining a DNA of a gene coding for the amino acid sequence of a desired protein with an implement for introducing into a predetermined microorganism, namely, a vector. The ability to produce the protein is given to the microorganism by introduction of the combined product (generally a plasmid), and culturing the microorganism to produce the protein as a result of the expression of that gene.
More efficient host-vector systems to be used in such a process had been investigated which include, in addition to E. coli, actinomycetes, Bacillus subtilis and yeasts.
Among these hosts, actinomycetes (e.g., bacteria belonging to the genus Streptomyces) are of particular interest. Actinoymcetes are particularly interesting since they can produce .beta.-lactam antibiotics, and moreover the expert in the art is well trained in the handling of these microorganisms especially in relation to safety, through production of antibiotics.
Some actinomycetes excrete protein products outside of the microorganism cells as in the case of extracellular .alpha.-amylase Such a phenomenon is speculated to occur by a mechanism in which a gene of the host microorganism, having specific gene information, at once produces a predetermined protein as a precursor containing in addition to the protein sequence a peptide chain which is called a signal peptide. The precursor protein is then secreted through the cell membrane outside the cell or into the periplasmic space, whereby the signal peptide chain is cleaved to produce a mature protein.
This so-called signal hypothesis, although presently widely accepted and based on various experimental facts, has not yet been clarified in regard to what specific amino acid sequence is present in the signal peptide when the host is an actinomycete.
As described above, the biotechnological production of a useful protein by microorganisms is accomplished through the expression of the gene for the protein introduced into the host microorganism via a vector. Accordingly, the incorporation during preparation of the plasmid of a gene coding for a signal peptide into the same plasmid carrying a gene fragment in an actinomycete would give rise to a precursor protein in which the signal peptide is fused with the desired protein, and the desired protein will be secreted outside the host actinomycete by the signal peptide.